Dynamic Power Angle Oscillation And Its Suppression Strategy Of Droop-Control Inverter

Under the background of the permeability of power electronics in power system is increasing,the dynamic time scale of inverters is at the same level as the dynamic time scale of line networks.The static power angle characteristics of traditional large power grids are no longer suitable for describing the power transmission process of inverters.Because of its small inertia and fast response,droop control inverters are easy to interact dynamically between double closed-loop control system and line network.Therefore,the dynamic of double closed-loop output impedance and line inductance flux linkage are realized in the power transmission process,and the power transmission characteristics show dynamic power angle characteristics.For dynamic power angle characteristics,this paper summarizes the existing literature,and finds that there are three aspects of dynamic power angle characteristics that need to be solved urgently.First,the dynamic power angle characteristics are not clearly defined.Most of the existing literatures understand the dynamic power angle characteristics literally,that is,the dynamic process of power transmission.Dynamic power angle characteristic is essentially a modeling problem,so the definition of dynamic power angle characteristic needs to start from the relationship between the models.Defining dynamic power angle characteristic can help us understand its position in the detailed time domain model of power transmission and the frequency of dynamic power angle oscillation.Secondly,the dynamic power angle characteristic model is improved.The existing literature only considers the influence of the line inductance flux on the dynamic power angle characteristics,but ignores the double closed-loop output impedance.The double closed-loop output impedance will reshape the impedance characteristics of the system,which will inevitably have an impact on the dynamic power angle characteristics.Thirdly,virtual resistance is mostly used to suppress dynamic power angle oscillation in existing literatures,but virtual resistance will increase the power coupling of sag-controlled inverters and affect the steady-state performance of the system.Therefore,it is necessary to find a damping strategy to suppress dynamic power angle oscillation without affecting the steady-state performance of the system.The main content of this paper is about the answers to the above three questions.With regard to dynamic power angle characteristics,this paper deeply analyses the idea of dynamic phasor modeling,based on the traditional static power angle characteristic which is the first-order steady-state phasor model of power transmission,the dynamic power angle characteristic is defined as the first-order dynamic phasor model of power transmission.This definition shows that the dynamic power angle characteristic reflects the low frequency dynamic form of power transmission,and the dynamic power angle oscillation is the low frequency oscillation mode in the droop control inverter system.With regard to the improvement of dynamic power angle characteristic modeling,the output impedance and equivalent circuit are used to improve the dynamic power angle characteristic model.The same results are obtained by the two methods,which verify the correctness of the dynamic power angle characteristic and dynamic power angle oscillation analyzed in this paper.As for the suppression strategy of dynamic power angle oscillation,this paper adopts transient virtual impedance strategy to suppress dynamic power angle oscillation.Transient virtual impedance strategy can suppress dynamic power angle oscillation without affecting the steady-state performance of the system and improve the dynamic process of the system.This paper also compares the two virtual strategies of transient virtual resistance and transient virtual reactance,and finds that transient virtual reactance can better modify the power transmission characteristics and restrain dynamic power angle oscillation.